Kinetics and product study of the self-reactions of allyl and allyl peroxy radicals at 296 K

Abstract

The laser flash photolysis technique, coupled with UV absorption spectroscopy, has been used to investigate the UV spectra and kinetics of reactions of the allyl radical (CH₂CHCH₂) and the allyl peroxy radical (CH₂CHCH₂O₂) at 296 K and total pressures near atmospheric. CH₂CHCH₂ radicals were generated by the 193 nm photolysis of hexa-1,5-diene–N₂ mixtures, or the 248 nm photolysis of allyl iodide–N₂ mixtures. The 193 nm photolysis of hexa-1,5-diene–O₂–N₂ mixtures was used to generate CH₂CHCH₂O₂. The low resolution spectrum of CH₂CHCH₂ was characterised in the range 210–232.5 nm. The absolute absorption cross-section near the maximum, σ(220 nm)=(5.8 ± 0.8)× 10^–17 cm² molecule^–1, calibrated relative to the loss of allyl iodide, is in good agreement with the single published determination. The observed time dependence of CH₂CHCH₂ in the various chemical systems allowed measurement of rate coefficients for the following reactions 2CH₂CHCH₂(+M)→ CH₂CHCH₂CH₂CHCH₂(+M)(10), CH₂CHCH₂+ l (+M)→ CH₂CHCH₂l (+M)(12), CH₂CHCH₂+ O₂(+M)→ CH₂CHCH₂O₂(+M)(7) The parameters obtained were k₁₀=(3.0 ± 0.5)× 10^–11, k₁₂=(1.6 ± 0.6)× 10^–10, and k₇=(6 ± 2)× 10^–13 cm³ molecule^–1 s^–1.

The UV absorption spectrum of CH₂CHCH₂O₂, characterised in the wavelength range 210–300 nm, is typical of an organic peroxy radical, with a peak cross-section of (6.2 ± 0.9)× 10^–18 cm² molecule^–1 at 235 nm. CH₂CHCH₂O₂ displayed second-order kinetic behaviour indicative of its removal via the self-reaction: 2CH₂CHCH₂O₂→ 2CH₂CHCH₂O + O₂(13a), → CH₂CHCHO + CH₂CHCH₂OH + O₂(13b) The observed rate coefficient, k_13obs=(1.1 ± 0.2)× 10^–12 cm³ molecule^–1 s^–1, is greater than the elementary coefficient, k₁₃, owing to secondary removal of CH₂CHCH₂O₂. The observed and elementary coefficients are related by the expression k_13obs=(1 +α₁₃)k₁₃, where α₁₃=k_13a/k₁₃. Associated long pathlength Fourier transform infrared (FTIR) measurements of the products of the 253.7 nm initiated photo-oxidation of allyl iodide, allowed the product channels of reaction (13) to be identified and quantified, and a value of α₁₃= 0.61 ± 0.07 was determined, leading to k₁₃=(6.8 ± 1.3)× 10³ cm^–13 molecule^–1 s^–1.

The kinetic and mechanistic data obtained for CH₂CHCH₂O₂, and those measured previously in this laboratory for HOCH₂CH₂O₂, are used to infer average rate coefficients for peroxy radicals (RO₂) formed in the OH-initiated oxidation of isoprene. A simple box model of the planetary boundary layer is used to demonstrate the potential impact of elevated concentrations of isoprene on ambient levels of OH, HO₂, RO₂, O₃ and NO_x.